Easy To Use Patents Search & Patent Lawyer Directory

At Patents you can conduct a Patent Search, File a Patent Application, find a Patent Attorney, or search available technology through our Patent Exchange. Patents are available using simple keyword or date criteria. If you are looking to hire a patent attorney, you've come to the right place. Protect your idea and hire a patent lawyer.

Biological-based polyurethanes and methods of making the same. The
polyurethanes are formed by reacting a biodegradable polyisocyanate (such
as lysine diisocyanate) with an optionally hydroxylated biomolecule to
form polyurethane. The polymers formed may be combined with ceramic
and/or bone particles to form a composite, which may be used as an
osteoimplant.

1. A polyurethane composite, comprising: a polyurethane formed by reaction
of a polyisocyanate with a hydroxylated or aminated material to form a
polyurethane polymer, wherein the composite comprises an included
material comprising a biomolecule, extracellular matrix component,
bioactive agent, small molecule, tissue-derived material, inorganic
ceramic, bone substitute, modified forms of the above, or a mixture of
any of the above.

2. (canceled)

3. (canceled)

4. The polyurethane composite of claim 1, wherein the modified form of the
included material has an increased surface concentration of hydroxyl or
amine groups with respect to the unmodified material.

5. The polyurethane composite of claim 1, wherein the included material is
the hydroxylated or aminated material.

6. (canceled)

7.-29. (canceled)

30. The polyurethane composite of claim 1, wherein the polyurethane
degrades at a rate sufficient to permit generation of new tissue at an in
vivo implantation site.

31.-36. (canceled)

37. A biodegradable polyurethane, formed by reaction of a polyisocyanate
with optionally hydroxylated biomolecules to form a polyurethane polymer,
wherein the optionally hydroxylated biomolecules comprise one or more of
polysaccharides and starches and one or more of lipids and phospholipids.

38. The polyurethane of claim 37, wherein the polymer is cross-linked.

39.-44. (canceled)

45. The polyurethane of claim 37, further comprising a included material
combined with the polyurethane.

46.-49. (canceled)

50. A non-biodegradable polyurethane, formed by reaction of a
polyisocyanate with optionally hydroxylated biomolecules to form a
polyurethane polymer, wherein the optionally hydroxylated biomolecules
comprise one or more of polysaccharides and starches.

51.-57. (canceled)

58. The polyurethane of claim 50, further comprising a included material
combined with the polyurethane.

59. The polyurethane of claim 58, wherein the included material comprises
a tissue-derived material, an inorganic ceramic, a bone substitute
material, modified forms of the above, or any combination of the above.

60. (canceled)

61. The polyurethane of claim 58, wherein the included material comprises
a composite, wherein the composite comprises a member of an inorganic
ceramic and a bone-derived material and a member of bovine serum albumin,
collagen, an extracellular matrix component, a synthetic polymer, or a
naturally derived polymer.

62. (canceled)

63. A method of making a polyurethane composite, comprising: reacting a
polyisocyanate with a hydroxylated or aminated material in the presence
of an included material to form a polyurethane polymer matrix having
particles of said included material embedded therein, wherein said
included material comprises a biomolecule, extracellular matrix
component, bioactive agent, small molecule, tissue-derived material,
inorganic ceramic, bone substitute, modified forms of the above, or a
mixture of any of the above.

64.-65. (canceled)

66. The method of claim 63, wherein the modified form of the included
material has an increased surface concentration of hydroxyl or amine
groups with respect to the unmodified material.

67. (canceled)

68. The method of claim 63, wherein the hydroxylated or aminated material
is the included material.

69.-71. (canceled)

72. The method of claim 63, wherein the biomolecule is selected from the
group consisting of phospholipids, fatty acids, cholesterols,
polysaccharides, lecithin, starches, collagen, and combinations and
modified forms of the above.

73. (canceled)

74. The method of claim 63, further comprising reacting the polyurethane
with a chain extender.

75. (canceled)

76. The method of claim 63, wherein reacting comprises: reacting the
polyisocyanate and the aminated or hydroxylated material to form a
prepolymer; mixing the prepolymer with the included material to form a
precomposite; and reacting the precomposite to form the polyurethane
composite.

84. The method of claim 63, further comprising increasing the cross-link
density of the polyurethane polymer matrix.

Description

[0001] This application is a continuation-in-part of U.S. application Ser.
No. 10/771,736, filed Feb. 4, 2004, which claims the benefit of U.S.
Provisional Application No. 60/444,759, filed Feb. 4, 2003, the entire
contents of both of which are incorporated herein by reference in its
entirety.

BACKGROUND OF THE INVENTION

[0002] Vertebrate bone is a composite material composed of hydroxyapatite,
collagen, and a variety of noncollagenous proteins, as well as embedded
and adherent cells. Vertebrate bone can be processed into an implantable
biomaterial, such as an allograft, for example, by removing the cells,
leaving behind the mineral and extracellular matrix. The processed bone
biomaterial can have a variety of properties, depending upon the specific
processes and treatments applied to it, and may incorporate
characteristics of other biomaterials with which it is combined. For
example, bone-derived biomaterials may be processed into load-bearing
mineralized grafts that support and integrate with the patient's bone or
may alternatively be processed into soft, moldable or flowable
demineralized bone biomaterials that have the ability to induce a
cellular healing response.

[0003] The use of bone grafts and bone substitute materials in orthopedic
medicine is well known. While bone wounds can regenerate without the
formation of scar tissue, fractures and other orthopedic injuries take a
long time to heal, during which the bone is unable to support physiologic
loading. Metal pins, screws, and meshes are frequently required to
replace the mechanical functions of injured bone. However, metal is
significantly stiffer than bone. Use of metal implants may result in
decreased bone density around the implant site due to stress shielding.
Furthermore, metal implants are permanent and unable to participate in
physiological remodeling.

[0004] Following implantation, the host's own bone remodeling capabilities
permit some bone grafts and certain bone substitute materials to remodel
into endogenous bone that in most cases is indistinguishable from the
host's own bone. In general, however, it is a limitation of allograft
bone that larger allografts do not completely remodel, and residual
allograft bone may persist at the graft site for many years or
indefinitely, potentially acting as a stress riser and a possible
fracture site. The use of bone grafts is further limited by the
availability of tissue with the appropriate shape and size, as well as
the desired mechanical strength and degradation rate.

[0005] U.S. Pat. No. 6,294,187, the contents of which are incorporated
herein by reference, describes methods for preparing composites including
allogenic bone for use in load bearing orthopedic applications. It is
desirable to increase the strength of bone-reinforced composites by
increasing the strength of the matrix material while retaining the
resorbable properties of the matrix. Furthermore, there is a need for a
novel resorbable polymer capable of synergistically interacting with bone
to make a true composite having mechanical characteristics of both bone
and polymer. There is also a need to develop resorbable polymers for the
production of bone/polymer composites where the polymer itself
contributes to osteointegration and remodeling of the composite. It is
also desirable to develop implants that do not elicit undesirable immune
responses from the recipient. There is also a need to provide composite
grafts of suitable shape and size that maximize the utility of the graft
tissue.

SUMMARY OF THE INVENTION

[0006] In one embodiment, the invention is a polyurethane composite
including a polyurethane formed by reaction of a polyisocyanate with a
hydroxylated or aminated material to form a polyurethane polymer. The
composite includes an included material including one or more of a
biomolecule, extracellular matrix component, bioactive agent, small
molecule, tissue-derived material, inorganic ceramic, bone substitute,
and modified forms of these. The included material may include a
composite including one or more of an inorganic ceramic and a
bone-derived material and one or more of bovine serum albumin, collagen,
an extracellular matrix component, a synthetic polymer, and a naturally
derived polymer. Modified forms may have an increased surface
concentration of hydroxyl or amine groups with respect to the unmodified
material. The included material may be the hydroxylated or aminated
material. The tissue-derived material may include bone, demineralized
bone, deorganified bone, or tissue derived from tendon, ligament,
cartilage, endodermis, small intestine, mucosa, skin, or muscle. At least
a portion of the bone or bone substitute may be lightly demineralized.
The biomolecule may be selected from phospholipids, fatty acids,
cholesterols, polysaccharides, lecithin, starches, collagen, and
combinations and modified forms of the above. The included material may
be selected from lectins, growth factors, immunosuppressives,
chemoattractants, antibiotics, and anticoagulants. The polyurethane may
have a wet compressive strength that exceeds the wet compressive strength
of the polyurethane alone. The polyurethane composite may degrade at a
rate sufficient to permit generation of new tissue at an in vivo
implantation site.

[0007] In another embodiment, the invention is biodegradable polyurethane
formed by reaction of a polyisocyanate with optionally hydroxylated
biomolecules to form a polyurethane polymer. The optionally hydroxylated
biomolecules include one or more of polysaccharides and starches and one
or more of lipids and phospholipids. An included material may be combined
with the polyurethane, for example, a tissue-derived material, an
inorganic ceramic, a bone substitute material, modified forms of the
above, or any combination. The included material may itself include a
composite.

[0008] In another embodiment, the invention is a non-biodegradable
polyurethane formed by reaction of a polyisocyanate with optionally
hydroxylated biomolecules to form a polyurethane polymer. The optionally
hydroxylated biomolecules include one or more of polysaccharides and
starches.

[0009] In another aspect, the invention is a method of making a
polyurethane composite. The method includes reacting a polyisocyanate
with a hydroxylated or aminated material in the presence of an included
material to form a polyurethane polymer matrix having particles of the
included material embedded therein. The included material includes a
biomolecule, extracellular matrix component, bioactive agent, small
molecule, tissue-derived material, inorganic ceramic, bone substitute,
modified forms of the above, or a mixture of any of these. Reacting may
include reacting the polyisocyanate and the aminated or hydroxylated
material to form a prepolymer, mixing the prepolymer with the included
material to form a precomposite, and reacting the precomposite to form
the polyurethane composite. Reacting the precomposite may include
cross-linking the prepolymer, reacting for a time period from about one
minute to about four hours, or exposing the polyisocyanate and the
hydroxylated or aminated material to a catalyst. The catalyst may include
a material selected from mild bases, strong bases, sodium hydroxide,
sodium acetate, tin, and triethylene diamine 1,4-diaza(2,2,2)
bicyclooctane. The method may further include increasing the cross-link
density of the polyurethane polymer matrix.

[0011] A more complete listing of bioactive agents and specific drugs
suitable for use in the present invention may be found in "Pharmaceutical
Substances: Syntheses, Patents, Applications" by Axel Kleemann and Jurgen
Engel, Thieme Medical Publishing, 1999; the "Merck Index: An Encyclopedia
of Chemicals, Drugs, and Biologicals", Edited by Susan Budavari et al.,
CRC Press, 1996, the United States Pharmacopeia-25/National Formulary-20,
published by the United States Pharmcopeial Convention, Inc., Rockville
Md., 2001, and the "Pharmazeutische Wirkstoffe," edited by Von Keemann et
al., Stuttgart/New York, 1987, all of which are incorporated herein by
reference.

[0012] The term "biocompatible," as used herein, is intended to describe
materials that, upon administration in vivo, do not induce undesirable
long term effects.

[0013] As used herein, "biodegradable," "bioerodable," or "resorbable"
materials are materials that degrade under physiological conditions to
form a product that can be metabolized or excreted. Biodegradable
materials are not necessarily hydrolytically degradable and may require
enzymatic action to fully degrade. Biodegradable materials also include
materials that are broken down within cells.

[0015] As used herein, the term "composite" refers to a mixture of two or
more different materials. In one embodiment, the two materials are a
polymer and an additional material. The additional material may include
several materials having different compositions, sizes, shapes, or other
characteristics. While the polymer may act as a binder to hold together
particles, fibers, or fragments of additional material(s), it is not
required that the polymer be fully interconnected throughout the
composite; neither is it assumed that the additional material is or is
not interconnected throughout the composite.

[0016] "Deorganified", as herein applied to matrices, particles, etc.,
refers to bone or cartilage matrices, particles, etc., that were
subjected to a process that removes at least part of their original
organic content. In some embodiments, at least 1%, 10%, 20%, 30%, 40%,
50%, 60%, 70%, 80%, 90%, or 99% of the organic content of the starting
material is removed. Deorganified bone from which substantially all the
organic components have been removed is termed "anorganic."

[0018] The term "osteogenic," as applied to the osteoimplant of this
invention, shall be understood as referring to the ability of the
osteoimplant to enhance or accelerate the ingrowth of new bone tissue by
one or more mechanisms such as osteogenesis, osteoconduction and/or
osteoinduction.

[0019] The term "polyisocyanate," as that term is used herein, encompasses
any chemical structure comprising two or more cyanate groups. A
"diisocyanate," as used herein, is a subset of the class of
polyisocyanates, a chemical structure containing exactly two cyanate
(--CN) groups. Similarly, a "polyol" contains two or more alcohol (--OH)
groups, while a "diol" contains exactly two alcohol groups, and a
"polyamine" contains two or more primary amine groups.

[0021] "Polypeptide", "peptide", or "protein": According to the present
invention, a "polypeptide," "peptide," or "protein" comprises a string of
at least two amino acids linked together by peptide bonds. The terms
"polypeptide", "peptide", and "protein", may be used interchangeably.
Peptide may refer to an individual peptide or a collection of peptides.
Inventive peptides preferably contain only natural amino acids, although
non-natural amino acids (i.e., compounds that do not occur in nature but
that can be incorporated into a polypeptide chain) and/or amino acid
analogs as are known in the art may alternatively be employed. Also, one
or more of the amino acids in an inventive peptide may be modified, for
example, by the addition of a chemical entity such as a carbohydrate
group, a phosphate group, a farnesyl group, an isofarnesyl group, a fatty
acid group, a linker for conjugation, functionalization, or other
modification, etc. In a preferred embodiment, the modifications of the
peptide lead to a more stable peptide (e.g., greater half-life in vivo).
These modifications may include cyclization of the peptide, the
incorporation of D-amino acids, etc. None of the modifications should
substantially interfere with the desired biological activity of the
peptide.

[0022] The terms "polysaccharide," "carbohydrate," "oligosaccharide," or
"starch" refer to a polymer of sugars. The terms "polysaccharide" and
"carbohydrate" may be used interchangeably to mean a sugar polymer of any
length. "Oligosaccharide" generally refers to a relatively low molecular
weight polymer, while "starch" typically refers to a higher molecular
weight polymer. The polymer may include natural sugars (e.g., glucose,
fructose, galactose, mannose, arabinose, ribose, and xylose) and/or
modified sugars (e.g., 2'-fluororibose, 2'-deoxyribose, and hexose).
Polysaccharides may or may not be crosslinked.

[0023] The term "polyurethane," as used herein, is intended to include all
polymers incorporating more than one urethane group (--NH--CO--O--) or
more than one area group (--NH--CO--NH--) in the polymer backbone.
Polymers containing only urea linkages, although technically termed
polyureas, are also referred to herein as polyurethanes.

[0024] The term "shaped," as applied to the osteoimplant herein, refers to
a determined or regular form or configuration, in contrast to an
indeterminate or vague form or configuration (as in the case of a lump or
other solid mass of no special form) and is characteristic of such
materials as sheet, plate, particle, sphere, hemisphere strand, coiled
strand, capillary network, film, fiber, mesh, disk, cone, portion of a
cone, pin, screw, tube, cup, tooth, tooth root, strut, wedge, portion of
wedge, cylinder, threaded cylinder, rod, hinge, rivet, anchor, spheroid,
ellipsoid, oblate spheroid, prolate ellipsoid, hyperbolic paraboloid, and
the like.

[0025] "Small molecule": As used herein, the term "small molecule" is used
to refer to molecules, whether naturally-occurring or artificially
created (e.g., via chemical synthesis), that have a relatively low
molecular weight. Typically, small molecules have a molecular weight of
less than about 5000 g/mol. Preferred small molecules are biologically
active in that they produce a local or systemic effect in animals,
preferably mammals, more preferably humans. In certain preferred
embodiments, the small molecule is a drug. Preferably, though not
necessarily, the drug is one that has already been deemed safe and
effective for use by the appropriate governmental agency or body. For
example, drugs for human use listed by the FDA under 21 C.F.R.
.sctn..sctn. 330.5, 331 through 361, and 440 through 460; drugs for
veterinary use listed by the FDA under 21 C.F.R. .sctn..sctn. 500 through
589, incorporated herein by reference, are all considered acceptable for
use in accordance with the present invention.

[0026] As utilized herein, the phrase "superficially demineralized" as
applied to bone particles refers to bone particles possessing at least
about 90 weight percent of their original inorganic mineral content. The
phrase "partially demineralized" as applied to the bone particles refers
to bone particles possessing from about 8 to about 90 weight percent of
their original inorganic mineral content, and the phrase "fully
demineralized" as applied to the bone particles refers to bone particles
possessing less than about 8, for example, less than about 1, weight
percent of their original inorganic mineral content. The unmodified term
"demineralized" as applied to the bone particles is intended to cover any
one or combination of the foregoing types of demineralized bone
particles.

[0027] Unless otherwise specified, all material proportions used herein
are in weight percent.

[0028] The phrase "wet compressive strength," as utilized herein, refers
to the compressive strength of the osteoimplant after the osteoimplant
has been immersed in simulated body fluid (SBF) for a minimum of 12
hours. Compressive strength is a well-known measurement of mechanical
strength and is measured using the procedure described herein.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS

[0029] In one embodiment, a polyurethane composite includes a polyurethane
formed by reaction of a polyisocyanate with a hydroxylated or aminated
material. The composite includes and included material, e.g., a
biomolecule, extracellular matrix component, bioactive agent, small
molecule, tissue-derived material, inorganic ceramic, bone substitute, a
composite of an inorganic ceramic with one or more of a tissue-derived
material, extracellular matrix material, and bovine serum albumin, or a
mixture thereof.

Components of a Polyurethane Composite

[0030] Polyurethanes are often formed by the reaction of a polyisocyanate
(such as a diisocyanate) with a polyol (such as a diol):

[0031] Polyurethanes may be straight chains or branched, and may have high
or low molecular weights. Polyurethanes may also contain urea linkages
formed by the reaction of an isocyanate with an amine. In an alternative
embodiment, polyurethanes are formed by reacting a polyol with an excess
of polyisocyanate to form a macropolyisocyanate prepolymer, following
which the prepolymer is reacted with a second polyol or polyamine to form
the polyurethane:

[0032] The R.sup.1, R.sub.2, and R.sub.3 groups provide great flexibility
in tailoring the mechanical and chemical properties of polyurethanes,
which may be made rigid, soft, plastic, and/or elastomeric by selection
of appropriate functional groups. The use of R groups having different
types of chemical linkages creates regions of the polyurethane that are
more and less flexible. For example, aromatic and polyaromatic R groups
increase the rigidity of that segment of the polymer, while alkane and
polyol chains are relatively flexible. The mixture of rigid, or hard,
with flexible, or soft, segments in a polyurethane results in a strong,
tough, elastomeric material. The ratio of hard and soft segments may be
adjusted to optimize the mechanical properties of the composite.

[0035] Exemplary chain extenders include but are not limited to
1,4-cyclohexane dimethanol, polyols of polyhydroxybutyrate and
polyhydroxyvalerate, polylactide, polyglycolide,
poly(lactide-co-glycolide), biocompatible diester diols and diurea diols,
1,4-butanediol, ethylene diamine, 4,4'-methylene bis (2-chloroaniline),
ethylene glycol, 3-hexyne-2,5-diol, 2-amino-1-butanol, and hexanediol.
One skilled in the art will recognize that other aromatic and aliphatic
diols and diamines may also be employed as chain extenders. The use of
biologically derived materials is discussed below.

[0036] In some embodiments, R.sub.1, R.sub.2, or R.sub.3 may include
alkyl, aryl, heterocycles, cycloalkyl, aromatic heterocycles,
multicycloalkyl, hydroxyl, ester, ether, carboxylic acid, amino,
alkylamino, dialkylamino, trialkylamino, amido, alkoxy, or ureido groups.
Alternatively or in addition, R.sub.1, R.sub.2, or R.sub.3 may also
include branches or substituents including alkyl, aryl, heterocycles,
cycloalkyl, aromatic heterocycles, multicycloalkyl, hydroxyl, ester,
ether, halide, carboxylic acid, amino, alkylamino, dialkylamino,
trialkylamino, amido, carbamoyl, thioether, thiol, alkoxy, or ureido
groups. Exemplary groups for use as R.sub.1, R.sub.2, or R.sub.3 also
include bioactive agents, biomolecules, and small molecules. Appropriate
polyurethanes also include those disclosed in U.S. Patent Publication No.
2005/0013793, the contents of which are incorporated herein by reference.

[0037] In some embodiments, polyurethane composites are formed by reacting
an appropriate polyisocyanate crosslinker (e.g., a diisocyanate) or
macropolyisocyanate prepolymer with an aminated or hydroxylated material
to form composites which may have osteogenic and/or osteoinductive
properties. Of course, the material may have both amine and hydroxyl
groups. The composites also incorporate an included material, for
example, a biomolecule, extracellular matrix component, bioactive agent,
small molecule, bone, bone substitute, tissue derived material, inorganic
ceramic, or a mixture of these. Details of traditional polyurethane
synthesis can be found, for example, in Lamba, et al., Polyurethanes in
Biomedical Applications, CRC Press, 1998, which is incorporated herein by
reference, and particularly in Chapter 2 of the above reference. The
hydroxylated or aminated material may serve as a polyol/polyamine in a
macropolyisocyanate, as a chain extender, or as any of R.sub.1, R.sub.2,
or R.sub.3.

[0038] Naturally derived materials may also be used as polyols or
polyamines and may serve as part of the macropolyisocyanate, the chain
extender, or both. In one embodiment, the hydroxylated or aminated
material is a biomolecule, for example, a lipid (e.g., phospholipid,
lecithin, fatty acid, trigyceride, or cholesterol) or polysaccharide
(e.g., oligosaccharide or amylase-resistant starches). A biomolecule for
use according to the techniques of the invention may be hydroxylated by
any method known to those skilled in the art if it does not already
possess sufficient reactive groups to carry out a reaction. For example,
lipids, including phospholipids, mono-, di-, and triglycerides, fatty
acids, and cholesterols may require addition of hydroxyl or amine groups
in order to carry out the polymerization reaction. In contrast, many
polysaccharides already have sufficient hydroxyl groups to polymerize
readily into a highly cross-linked polymer.

[0039] The hydroxylated or aminated material may also include intact
extracellular matrix (ECM), its components, alone or in combination, or
modified or synthetic versions thereof. These materials may be treated to
increase the concentration of hydroxyl and/or amino groups, especially
the surface concentration of these groups. For example, collagen may be
decross-linked or treated with lysyl oxidase. Lysyl oxidase converts the
terminal amino groups of lysine to aldehydes, which may then be reduced.
Alternatively or in addition, the biomolecule, or ECM component, or
tissue may be aminated. The amino groups will be incorporated into the
polymer through a urea linkage. Of course, many ECM derived materials
already contain primary amino groups. Exemplary extracellular matrix
components suitable for use with the invention include but are not
limited to collagen, laminin, elastin, proteoglycans, reticulin,
fibronectin, vitronectin, glycosaminoglycans, and other basement membrane
components. Various types of collagen (e.g., collagen Type I, collagen
Type II, collagen Type IV, etc., as well as collagen derived or denatured
materials such as gelatin) are suitable for use with the invention.
Collagens may be used in fiber, gel, or other forms. Sources for
extracellular matrix components include, but are not limited to, skin,
tendon, intestine and dura mater obtained from animals, transgenic
animals and humans. Extracellular matrix components are also commercially
available, for example, from Becton Dickinson. Collagenous tissue can
also be obtained by genetically engineering microorganisms to express
collagen as described, e.g., in U.S. Pat. No. 5,243,038, the entire
contents of which are incorporated herein by reference. Procedures for
obtaining and purifying collagen are well known in the art and typically
involve acid or enzyme extraction as described, e.g., in U.S. Pat. No.
5,263,984, the contents of which are incorporated by reference herein.
Exemplary synthetic ECM analogs include RGD-containing peptides,
silk-elastin polymers produced by Protein Polymer Technologies (San
Diego, Calif.) and BioSteel.TM., a recombinant spider silk produced by
Nexia Biotechnologies (Vaudrevil-Dorion, QC, Canada). Various types of
collagen (e.g., collagen Type I, collagen Type II, collagen Type IV) are
also suitable for use with embodiments of the invention.

[0040] Tissues, including but not limited to xenograft, allograft, or
autograft tissues, including non-bony tissues and bone-derived tissues,
may be used with the invention. The preparation of bone pieces and
particles for incorporation into composites is discussed below. Non-bony
tissues suitable for use with the invention include connective tissue
such as tendon, ligament, cartilage, endodermis, small intestinal
submucosa, skin, and muscle. The tissues may be excised and cut into a
plurality of elongated fragments or particles employing methods known in
the art. Reduction of the antigenicity of allogeneic and xenogeneic
tissue can be achieved by treating the tissues with various chemical
agents, e.g., extraction agents such as monoglycerides, diglycerides,
triglycerides, dimethyl formamide, etc., as described, e.g., in U.S. Pat.
No. 5,507,810, the contents of which are incorporated by reference
herein. Small intestine submucosa tissue can be obtained and processed as
described in U.S. Pat. No. 4,902,508, the contents of which are
incorporated by reference herein. In summary, intestinal tissue is
abraded to remove the outer layers, including both the tunica serosa and
the tunica muscularis and the inner layers, including at least the
luminal portion of the tunica mucosa. The resulting material is a
whitish, translucent tube of tissue approximately 0.1 mm thick, typically
consisting of the tunica submucosa with the attached lamina muscularis
mucosa and stratum compactum. The tissue may be rinsed in 10% neomycin
sulfate before use. Tissues may be modified by demineralization,
amination, or hydroxylation before use. For example, lysine groups may be
modified with lysyl oxidase as described above.

[0041] Ceramics, including calcium phosphate materials and bone substitute
materials, may also be exploited for use as particulate inclusions or as
the hydroxylated or aminated material. Exemplary inorganic ceramics for
use with the invention include calcium carbonate, calcium sulfate,
calcium phosphosilicate, sodium phosphate, calcium aluminate, calcium
phosphate, hydroxyapatite, .alpha.-tricalcium phosphate, dicalcium
phosphate, .beta.-tricalcium phosphate, tetracalcium phosphate, amorphous
calcium phosphate, octacalcium phosphate, and BIOGLASS.TM., a calcium
phosphate silica glass available from U.S. Biomaterials Corporation.
Substituted CaP phases are also contemplated for use with the invention,
including but not limited to fluorapatite, chlorapatite, Mg-substituted
tricalcium phosphate, and carbonate hydroxyapatite. Additional calcium
phosphate phases suitable for use with the invention include those
disclosed in U.S. Pat. Nos. RE 33,161 and RE 33,221 to Brown et al.; U.S.
Pat. Nos. 4,880,610; 5,034,059; 5,047,031; 5,053,212; 5,129,905;
5,336,264; and 6,002,065 to Constantz et al.; U.S. Pat. Nos. 5,149,368;
5,262,166 and 5,462,722 to Liu et al.; U.S. Pat. Nos. 5,525,148 and
5,542,973 to Chow et al., U.S. Pat. Nos. 5,717,006 and 6,001,394 to
Daculsi et al., U.S. Pat. No. 5,605,713 to Boltong et al., U.S. Pat. No.
5,650,176 to Lee et al., and U.S. Pat. No. 6,206,957 to Driessens et al,
and biologically-derived or biomimetic materials such as those identified
in Lowenstam H A, Weiner S, On Biomineralization, Oxford University
Press, 1989, incorporated herein by reference. The composite may contain
between about 5 and 80% bone-derived or other ceramic material, for
example, between about 60 and about 75%.

[0042] In another embodiment, a composite material may be reacted with a
macropolyisocyanate to form a polyurethane composite. For example,
inorganic ceramics such as those described above or bone-derived
materials may be combined with proteins such as BSA, collagen, or other
extracellular matrix components such as those described above to form a
composite. Alternatively or in addition, inorganic ceramics or
bone-derived materials may be combined with synthetic or
naturally-derived polymers to form a composite using the techniques
described in our co-pending applications Ser. No. 10/735,135, filed Dec.
12, 2003, Ser. No. 10/681,651, filed Oct. 8, 2003, and Ser. No.
10/639,912, filed Aug. 12, 2003, the contents of all of which are
incorporated herein by reference. These composites may be lightly
demineralized as described below to expose the organic material at the
surface of the composite before they are formed into polyurethane
composites according to the teachings of the invention.

[0043] Particulate materials for use with an embodiment of the invention
may be modified to increase the concentration of amino or hydroxyl groups
at their surfaces using the techniques described elsewhere herein.
Particulate materials may also be rendered more reactive through
treatment with silane coupling reagents, such as those described in our
co-pending application, published as U.S. Patent Publication No.
20050008620, the entire contents of which are incorporated herein by
reference. Coupling agents may be used to link polyisocyanate, polyamine,
or polyol molecules to the particle or simply to attach individual amine,
hydroxyl or isocyanate groups. The linked molecules may be monomeric or
oligomeric.

[0044] When the hydroxylated or aminated material is difunctional,
reaction with a diisocyanate generally produces a polyurethane with
minimal crosslinking. Such polymers are generally thermoplastic and
readily deformable and may be subjected to strain-induced crystallization
for hardening. In contrast, if at least some reactants include at least
three active groups participating in the reaction, then the polymer will
generally be heavily cross-linked. Such polymers are often thermosets and
tend to be harder than polymers with low cross-linking. In addition,
their mechanical properties tend to be less dependent on how they are
processed, which may render them more machinable. Cross-linking may also
be controlled through the choice of catalyst. Exemplary catalysts include
mild bases, strong bases, sodium hydroxide, sodium acetate, tin, and
triethylene diamine-1,4-diaza(2,2,2) bicyclooctane. Tin and other metal
carboxylates promote branching and crosslinking during polyurethane
formation. The stoichiometry and temperature of the reaction may also be
adjusted to control the extent of crosslinking.

[0045] Because the reaction process combines an isocyanate with a
biomolecule or other biological or biocompatible material, many possible
breakdown products of the polymer according to certain embodiments are
themselves resorbable. In one embodiment, byproducts of enzymatic
degradation, dissolution, bioerosion, or other degradative processes are
biocompatible. These byproducts may be utilized in or may be metabolites
of any cellular metabolic pathway, such as but not limited to cellular
respiration, glycolysis, fermentation, or the tricarboxylic acid cycle.
In one embodiment, the polyurethanes of the invention are themselves
enzymatically degradable, bioerodable, hydrolyzable, and/or
bioabsorbable. Thus, when an osteoimplant is formed from the materials of
the invention, it can be slowly replaced by the ingrowth of natural bone
as the implant degrades. This process of osteogenesis may be accelerated,
for example, by the addition of bioactive agents. Such bioactive agents
may be incorporated into the polymer structure, either as backbone
elements or as side groups, or they may be present as solutes in the
solid polymer or as non-covalently bonded attachments. In any case, they
may be gradually released as the polyurethane degrades. The rate of
release may be tailored by modifying the attachment or incorporation of
the bioactive agents into the polymer. Bioactive agents that may be used
include not only agents having osteogenic properties, but also agents
having other biological properties such as immunosuppression and
chemoattraction. Exemplary bioactive agents include bone stimulating
peptides such as RGD, bone morphogenic proteins, and other growth
factors. Lectins are a class of particular interest for incorporation
into the present polymers, especially when the polymers comprise
carbohydrates, which bond readily to lectins.

[0046] For certain applications, it may be desirable to create foamed
polyurethane, rather than solid polyurethane. While typical foaming
agents such as hydrochlorofluorocarbons, hydrofluorocarbons, and pentanes
may not be biocompatible for many systems, other biocompatible agents may
be used. For example, Zhang et al. have found that water may be an
adequate foaming agent for a lysine diisocyanate/PEG/glycerol
polyurethane (see Zhang, et al., "Three-dimensional biocompatible
ascorbic acid-containing scaffold for bone tissue engineering," supra)
and may also be used to cause foaming in other polyurethanes. Other
foaming agents include dry ice or other agents that release carbon
dioxide or other gases into the composite. Alternatively, or in addition,
salts may be mixed in with the reagents and then dissolved after
polymerization to leave behind small voids.

[0047] Whether foamed or solid, polyurethanes may be formed with an
additional, included material. Exemplary included materials include but
are not limited to bone-derived tissue, non-bone derived tissue, and
ceramics and bone substitute materials. In some embodiments, settable
osteogenic materials (e.g. A-BSM, available from ETEX Corp, Cambridge,
Mass., Norian SRS, (Skeletal Repair System) available from Norian Corp,
Cupertino, Calif., or Dynaflex, available from Citagenix) is included in
the polyurethane composite. These materials may bond strongly to the
isocyanates used in forming the polymer, since they contain or may be
modified to contain significant numbers of active hydroxyl groups. Thus,
it may be preferred in some embodiments to first mix the included
material with the hydroxylated or aminated material, before addition of
the isocyanate. Nevertheless, it is also within the scope of the
invention to mix the additional material into already-combined
hydroxylated or aminated material and isocyanate, or to combine all three
components simultaneously. The amount of included material in the
composite will vary depending on the desired application, and practically
any amount of material, for example, at least 10, at least 30, at least
50, or at least 70% of the composite may be formed from the included
material.

[0048] Of course, the included material may serve as the hydroxylated or
aminated material. That is, materials such as biomolecules, extracellular
matrix components, bioactive agents, small molecules, tissue-derived
materials, inorganic ceramics, bone substitutes, and composites, such as
those described above, of inorganic ceramics or bone derived materials
with synthetic or naturally derived materials, extracellular matrix
material, and bovine serum albumin may react with the polyisocyanate to
form a polyurethane composite. In some embodiments, it may be desired to
form a prepolymer of isocyanate-terminated polyurethane oligomers and
react these with the included material to form the composite to add
flexibility to the polymer matrix.

Preparation of Bone for Incorporation into Composites

[0049] In one embodiment, the bone particles are produced from fully
mineralized human cortical bone. Bone particles for use in the composites
of the invention may also be obtained from cortical, cancellous, and/or
corticocancellous bone which may be of autogenous, allogenic and/or
xenogeneic origin and may or may not contain cells and/or cellular
components. Porcine and bovine bone are particularly advantageous types
of xenogeneic bone tissue that may be used individually or in combination
as sources for the bone particles. Bone particles for use in the
composites of the invention may be any shape including, for example,
irregular particulates, plates, fibers, helices and the like. Exemplary
fibers may have a length between 0.05 and 500 mm, for example, between 5
and 100 mm, a thickness between 0.01 and 2 mm, for example, between 0.05
and I mm, and a width between 0.1 and 20 mm, for example, between 2 and 5
mm. As described herein, bone fibers are particles having at least one
aspect ratio of 2:1 or greater. In some embodiments, bone fibers may have
a ratio of width to length of at least 5:1, 10:1, 15:1, 25:1, 50:1,
200:1, or 500:1.

[0050] Bone particles may be obtained by milling or shaving sequential
surfaces of an entire bone or relatively large section of bone. A
non-helical, four fluted end mill may be used to produce fibers having
the same orientation as the milled block. Such a mill has straight
grooves, or flutes, similar to a reamer, rather than helical flutes
resembling a drill bit. During the milling process, the bone may be
oriented such that the natural growth pattern (along the long axis) of
the piece being milled is along the long axis of the end mill of the
milling machine. Multiple passes of the non-helical end mill over the
bone results in bone particles having a long axis parallel to that of the
original bone (FIGS. 1, 2). Bone particles and fibers with different
sizes, dimensions, and aspect ratios may be obtained by adjusting the
milling parameters, including sweep speed, bit engagement, rpm, cut
depth, etc.

[0051] Elongated bone fibers may also be produced using the bone
processing mill described in commonly assigned U.S. Pat. No. 5,607,269,
the entire contents of which are incorporated herein by reference. Use of
this bone mill results in the production of long, thin strips which
quickly curl lengthwise to provide helical, tube-like bone particles. A
great variety of particle shapes (curled fiber, uncurled fiber, ribbon,
ship, short fiber, etc.) may be achieved by varying the speed, feed,
attack depth, engagement length and bit design. Elongated bone particles
may be graded into different sizes to reduce or eliminate any less
desirable size(s) of particles that may be present. In overall
appearance, particles produced using this mill may be described as
filaments, fibers, threads, slender or narrow strips, etc. In alternative
embodiments, bone fibers and particles may be produced by chipping,
rolling, fracturing with liquid nitrogen, chiseling or planeing,
broaching, cutting, or splitting along the axis (e.g., as wood is split
with a wedge).

[0052] The bone fibers may be sieved into different diameter sizes to
eliminate any less desirably sized fibers or more evenly dimensioned
particles that may be present. In one embodiment, fibers collected from
the milling machine may be lyophilized and manually sieved into a range
of 3-6 mm in length. Fiber length may be independent of cross-sectional
dimension and may be modified by adjusting the bit engagement length, the
length of the bit in contact with the bone during the milling operation.
Fibers may be an inch long or greater and may be as short as desired,
depending on the desired aspect ratio. Fibers less than 50 .mu.m long may
increase the likelihood of inflammation depending on the tissues and how
the implant degrades. In some instances, particles or fibers of this size
may be advantageously included to promote faster bone healing by
eliciting a mild inflammatory response. Larger fibers may be further
broken into smaller fibers by manually rolling them between the thumb and
fingers or by an equivalent automated method and then sieved again to
select the proper size fibers. Alternatively, fibers may be broken into
smaller fibers by pressing or rolling. The resulting fibers may have an
aspect ratio of 5:1 to 10:1. Broader or narrower fibers may be obtained
by changing sieve grate sizes.

[0053] Larger bone pieces may also be incorporated into composites
produced using the techniques of the invention. For example, fragments or
pieces of bone may be employed. Exemplary bone pieces include portions of
the diaphysis or metaphysis of the long bones, e.g., femur, tibia, ulna,
humerus, fibula, and radius, the phalanges or portions thereof, or large
pieces cut from bones such as the pelvis or jaw. Such pieces may include
transverse or longitudinal sections, portions of sections, or arbitrarily
shaped bits. Alternatively or in addition, bone may be cut into shapes
that are used for orthopedic implants or assembled to form an implant
before being combined with monomer or polymer. Exemplary shapes are shown
in FIG. 1.

[0054] The bone particles or pieces are optionally demineralized in
accordance with known and conventional procedures in order to reduce
their inorganic mineral content. Demineralization methods remove the
inorganic mineral component of bone, for example by employing acid
solutions. Such methods are well known in the art, see for example,
Reddi, et al., Proc. Nat. Acad. Sci., 1972, 69:1601-1605, the contents of
which are incorporated herein by reference. The strength of the acid
solution, the shape of the bone particles and the duration of the
demineralization treatment will determine the extent of demineralization.
Reference in this regard may be made to Lewandrowski, et al., J. Biomed.
Mater. Res., 1996, 31: 365-372, the contents of which are also
incorporated herein by reference. Bone particles may also be partially
demineralized. For example, bone particles may be demineralized to a
depth greater than 100 .mu.m, for example, between 100 and 5000 .mu.m,
between 150 .mu.m and 2000 .mu.m, or between 200 and 1000 .mu.m.

[0055] In an exemplary demineralization procedure, the bone particles are
subjected to an optional defatting/disinfecting step, followed by an acid
demineralization step. An exemplary defatting/disinfectant solution is an
aqueous solution of ethanol. Ordinarily, at least about 10 to about 40
percent by weight of water (i.e., about 60 to about 90 weight percent of
defatting agent such as alcohol) is present in the defatting/disinfecting
solution to optimize lipid removal and disinfection and processing time.
An exemplary concentration range of the defatting solution is from about
60 to about 85 weight percent alcohol, for example, about 70 weight
percent alcohol. Following defatting, the bone particles are immersed in
acid over time to effect their demineralization. The acid may also
disinfect the bone by killing viruses, vegetative microorganisms, and/or
spores. Acids that may be employed in this step include inorganic acids
such as hydrochloric acid and organic acids such as peracetic acid.
Alternative acids are well known to those skilled in the art. After acid
treatment, the demineralized bone particles are rinsed with sterile water
to remove residual amounts of acid and raise the pH. The bone particles
may be dried, for example, by lyophilization, before being incorporated
into the composite. The bone particles may be stored under aseptic
conditions until they are used or sterilized using known methods shortly
before incorporation into the composite. Additional demineralization
methods are well known to those skilled in the art, for example, the
method cited in Urist M R, A morphogenetic matrix for differentiation of
bone tissue, Calcif Tissue Res. 1970; Suppl:98-101 and Urist M R, Bone:
formation by autoinduction, Science. Nov. 12, 1965; 150(698):893-9, the
contents of both of which are incorporated herein by reference.

[0056] In an alternative embodiment, surfaces of bone particles may be
lightly demineralized according to the procedures in our commonly owned
U.S. patent application Ser. No. 10/285,715, published as U.S. Patent
Publication No. 20030144743, the entire contents of which are
incorporated herein by reference. Even minimal demineralization, for
example, of less than 5% removal of the inorganic phase, increases the
hydroxylation of bone fibers and the surface concentration of amine
groups. Demineralization may be so minimal, for example, less than 1%,
that the removal of the calcium phosphate phase is almost undetectable.
Rather, the enhanced surface concentration of reactive groups defines the
extent of demineralization. This may be measured, for example, by
titrating the reactive groups. In one embodiment, in a polymerization
reaction that utilizes the exposed allograft surfaces to initiate a
reaction, the amount of unreacted monomer remaining may be used to
estimate reactivity of the surfaces. Surface reactivity may be assessed
by a surrogate mechanical test, such as a peel test of a treated coupon
of bone adhering to a polymer. Alternatively or in addition, a portion of
the surface of the bone particles may be so demineralized.

[0057] Mixtures or combinations of one or more of the above types of bone
particles can be employed. For example, one or more of the foregoing
types of demineralized bone particles can be employed in combination with
nondemineralized bone particles, i.e., bone particles that have not been
subjected to a demineralization process. Anorganic bone, bone from which
at least a portion of the organic content has been removed, may also be
employed, either alone or in combination with other bone derived or
non-bone derived materials. The demineralized bone particles may behave
as short fibers in the composite, acting to increase fracture toughness.
The nondemineralized bone particles may behave as ceramic inclusions,
increasing the compressive strength of the composite. Nondemineralized
bone is itself a fiber-reinforced composite, which may increase the
bending and tensile stress the composite can withstand before the bone
particles break. Superficial demineralization produces particles
containing a mineralized core. Particles of this type may behave as
non-demineralized particles in the composite, depending on the degree of
demineralization. Minimally demineralized bone and other partially
demineralized bone pieces may be combined to form composite sandwiches
having carefully tailored mechanical properties. Slabs of bone
demineralized to the same or different degrees may be sandwiched using a
polyurethane using the techniques provided by the invention.
Multi-layered structures may also be produced. Bone may also be processed
to remove a portion of the organic content (e.g., deorganified bone), or
commercially available products such as BIO-OSS.TM. available from
Osteohealth, may be used.

Preparation of Polyurethane Composites

[0058] The hydroxylated or aminated material, any included material, and
the polyisocyanate or macropolyisocyanate may be combined using standard
composite processing techniques. The techniques described in our
co-pending U.S. patent applications Ser. No. 10/639,912, filed Aug. 12,
2003, and Ser. No. 10/735,135, filed Dec. 12, 2003, and those disclosed
in our co-pending application entitled "Injectable and Settable Bone
Substitute Material", filed on even date herewith, may also be used to
prepare the polyurethane and implant it into a tissue site.

[0059] For example, the components may be combined and injection molded,
injected, extruded, laminated, sheet formed, foamed, or processed using
other techniques known to those skilled in the art. Reaction injection
molding methods, in which a polyisocyanate and a polyol are separately
charged into a mold under precisely defined conditions, may be employed.
For example, the included material may be added to one of the precursor
materials, or it may be separately charged into the mold and the
precursor materials added afterwards. Careful control of the relative
amounts of the various components and the reaction conditions may be
desired to limit the amount of unreacted material in the composite.
Post-cure processes known to those skilled in the art may also be
employed. A partially polymerized precursor may be more completely
polymerized or cross-linked after combination with the hydroxylated or
aminated material or the included material. Alternatively or in addition,
porosity may be introduced to the composite using foaming processes,
e.g., by adding a porogen before or during polymerization, or by limiting
the amount of water in the reaction vessel and applying vacuum during
polymerization.

[0060] Alternatively or in addition, the various components may be
combined and pressed in a Carver press or other compression molding
device. Exemplary pressures include pressures ranging from about 1 psi to
about 30,000 psi, including around 1,000 psi, around 10,000 psi, around
15,000 psi, around 20,000 psi, or around 25,000 psi. For melt casting
applications, heat may be applied in conjunction with the pressure. In
some embodiments, any temperature between 20.degree. C. and about
300.degree. C. may be used. One skilled in the art will recognize that
higher temperatures may be needed, and that the processing temperature
may be optimized to allow the polymer to be processed without damaging
other components of the composite. The particular pressure to be used
will depend on the materials being pressed.

[0061] In one embodiment, the components are tabletted together before
being charged into a mold. For example, the components may be combined
and fed into a tabletting apparatus. Any pharmaceutical tablet press may
be used, for example, the Minipress available from Globe Pharma, Inc., of
New Brunswick, N.J. The tablets enable a more uniform distribution of
particulate included materials or particulate aminated/hydroxylated
materials in the polymer matrix. The tabletting process produces tabs of
a relatively uniform mass and composition. One or more tablets may be
charged into a mold to be pressed into a composite.

Post-Polymerization Processing

[0062] The surface of the composite may be modified after the polyurethane
is polymerized. Some processing methods cause the surface of the
composite to be primarily composed of polymer matrix rather than any
included material. Abrasion methods are useful for exposing particulate
included materials and provide surface roughness. Machining or cutting
the composite will also expose particulates. Surface roughening may be
accomplished mechanically, for example, through sanding, tumbling with a
hard material such as sand, or the use of a pulsatile wave (e.g., the
composite is conveyed above a liquid bath, and waves pulse the liquid
into crests that contact the material). The desired surface texture may
also be achieved using other machining methods, including but not limited
to grinding, milling, cutting, broaching, drilling, laser etching, water
cutting, and sand blasting. Chemical treatments may be used as well.
Implants containing hydrolytically degradable polymers may be treated
with water to pre-degrade the surface before implantation. The surface of
the composite may also be modified to postpone cellular ingrowth. For
example, the composite may be coated with a rapidly degradable or soluble
material, or regions may be masked so that the polyurethane polymer is
not exposed in certain regions during abrasive grinding, tumbling,
sanding, etc. operations. The rate at which the surface of the composite
is exposed may be adjusted such that the included material is revealed at
a particular point in the healing cascade.

[0063] Of course, the composite may also be machined. In one embodiment,
the composite is machined into a block which can be completely
infiltrated by tissue within a predetermined time period. Alternatively,
the composite may be machined into any desired shape and size. Exemplary
shapes include sheet, plate, particle, sphere, hemisphere, strand, coiled
strand, capillary network, film, fiber, mesh, disk, cone, portion of a
cone, pin, screw, tube, cup, tooth, tooth root, bone, portion of bone,
strut, wedge, portion of wedge, cylinder, threaded cylinder, rod, hinge,
rivet, anchor, spheroid, ellipsoid, oblate spheroid, prolate ellipsoid,
hyperbolic paraboloid. Composites may also be formed into the shape of a
bone or a portion of a bone. Exemplary bones whose shape the composite
may match in whole or in part (and which may be repaired or replaced
using the techniques of the invention) include ethmoid, frontal, nasal,
occipital, parietal, temporal, mandible, maxilla, zygomatic, cervical
vertebra, thoracic vertebra, lumbar vertebra, sacrum, rib, sternum,
clavicle, scapula, humerus, radius, ulna, carpal bones, metacarpal bones,
phalanges, incus, malleus, stapes, ilium, ischium, pubis, femur, tibia,
fibula, patella, calcaneus, tarsal and metatarsal bones. In another
embodiment, the composite is formed as a plate or similar support,
including but not limited to an I-shape to be placed between teeth for
intra-bony defects, a crescent apron for single site use, a rectangular
bib for defects including both the buccal and lingual alveolar ridges,
neutralization plates, spoon plates, condylar plates, clover leaf plates,
compression plates, bridge plates, wave plates, etc. Partial tubular as
well as flat plates may be fabricated using the techniques provided by
the invention. Composites may be molded into any of these shapes as well,
obviating a machining step or reducing the amount of machining needed.

[0064] In an alternative embodiment, bores or holes may be introduced into
the composite. Such holes may be drilled after the composite is formed.
Alternatively or in addition, the holes may be molded into place to
introduce holes into the composite. Such holes may be used to provide an
anchor for sutures, screws, or other fasteners, or as access channels for
cellular penetration and bone remodeling. Of course, cells will also
migrate into the hole after implantation.

[0065] The polyurethane composites of the invention may have a sufficient
wet compressive strength to provide mechanical stability for an
osteoimplant during healing. As the material degrades, it may retain some
mechanical strength, for example, having at least 25 MPa, residual
strength after 6 months in vivo. Alternatively, they may maintain at
least 70% of their original strength after 8 or 24 weeks. In one
embodiment, the composite exhibits stiffness in excess of 500 MPa,
compressive strength in excess of 25 MPa, torsional strength in excess of
20 MPa, and bending strength exceeding 50 MPa. In another embodiment, the
composite exhibits compressive strength exceeding 100 MPa, torsional
strength exceeding 75 MPa, stiffness exceeding 5 GPa, and bending
strength exceeding 150 MPa. For example, a bone void filler can transform
quickly and need not have high mechanical strength, while a lumbar
interbody implant may need to exhibit substantially higher compressive
and fatigue strength as it is transformed. In some embodiments, a
property of the polyurethane, e.g., mass, stiffness, torsional strength,
bending strength, etc., may degrade at a rate of about 5%, about 10% or
about 25% of original implant weight per month after implantation in
vivo. In many embodiments, this degradation will be accompanied by an
increase in the amount of or development of the mechanical properties of
bony tissue at the implant site, thereby maintaining the overall
mechanical strength of the material at the site. In some embodiments,
especially where the expected loads on the implant are expected to be
less (e.g, cranial implants), the transformation rate of the implant may
be increased by adding porosity to the implant using the methods
discussed above.

[0066] The mechanical properties desired for the composite and implants
fabricated from the composite may depend on the application in which the
implant will be used. One skilled in the art will understand how the
compressive strength of the composite should be varied for other
applications. Creep rates of less than 1% per 24 hours at 25 MPa (wet)
after 24 hours or 10% per 24 hours after 3 months are desirable. In
addition, humans apply about 1 to 1.1. million cycles of loading per
year, from the activities of daily living (Morlock M, et al., Duration
and frequency of every day activities in total hip patients. J. Biomech,
(2001) 34:873-81). By assessing the healing time and adding a factor of
safety, a desired fatigue period can be assessed. An exemplary target for
interbody spinal applications is 10 million cycles at 1.5 kN or 5 million
cycles at 3 kN. The applied stresses depend upon implant geometry but may
range from, e.g., about 5 to about 30 MPa. Fatigue loading targets for
other orthopedic applications may be as great or less. Maximum resolved
shear and tensile strengths of 3 MPa or greater and absolute maximum
resolved compressive strengths of 3 MPa or greater are also desirable.
However, even if these mechanical properties are not present in the
polymer or composite, the polymers and composites of the invention can be
combined with other materials to modify their mechanical properties. In
some embodiments, the mechanical strength, elastic modulus, and
anisotropic properties of the implant can be tailored by adjusting the
polymer chain length distribution, side chain length, degree of
cross-linking, and/or physical processing.

EXAMPLES

Example #1

[0067] To determine the compressive strength of a composite implant made
of approximately 66.6% bone and 33.3% castor bean polyurethane resin; 20
grams of bovine bone powder (particle size 120 .mu.m-500 .mu.m) were
combined with a two part polyurethane (Doctors Research Group, Plymouth
Conn. and described in "Vegetal Polyurethane Resin Implant Cranioplasty.
Experimental Studies in Rabbits" by Luiz Fernando Francisco, Sao Jose do
Rio Preto, 1998, which is incorporated herein by reference). Firstly,
6.10 grams of liquid comprising a polyisocyanate terminated molecule
"prepolymer" were combined with 3.60 grams of a liquid comprising castor
bean oil fatty acid triglyceride "diol". Next, bone particles were
gradually mixed into the polyurethane solution, until the bone appeared
well coated. The mixture was then packed by hand into three 5 cc syringes
(packed with light hand pressure). The samples were then set aside to
polymerize over a 48-hour period at room temperature. After
polymerization was complete, the samples were removed from the syringes
and cut to length (approx. 16 mm). Of the 4 samples tested; 2 were tested
dry, while two were hydrated in Simulated Body Fluid (SBF) for 24 hours
and tested wet. SBF solution contained 7.992-7.998 g NaCl, 0.2230-0.2243
g KCl, 0.2275-0.2289 g K.sub.2HPO.sub.4.3H.sub.2O, 0.3041-0.3059 g
MgCl.sub.2.6H.sub.2O, 36-40 ml HCl (1N), 0.3665-0.3687 g
CaCl.sub.2.2H.sub.2O, 0.0708-0.0712 g Na.sub.2SO.sub.4, 0.3517-0.3539 g
NaHCO.sub.3, and deionized water to make 1000 ml, adjusted to a pH of
7.2-7.4 by a buffer solution of tris(hydroxymethyl)aminomethane. The
results of mechanical static compression tests using the Bionix MTS 858
(Edin Prarrie Minn.) are shown in column 5 of Table 1. Results indicated
a slight decrease in compressive strength (of about 7%) with the hydrated
implants compared to the compressive strength of the dry implants, but
load bearing capacity was still considered acceptable for use as an
implant.
TABLE-US-00001
TABLE 1
Compressive
Sample Length (mm) Diameter (mm) Weight (g) Strength (MPa)
A-Dry 16.74 11.85 2.70 72
B-Dry 16.58 11.84 2.64 72
C-Wet 16.68 11.87 2.63 66
D-Wet 16.70 11.87 2.63 67

Example #2

[0068] To determine the compressive strength of an implant made of 100%
two-part castor bean polyurethane resin, (Doctors Research Group,
Plymouth Conn. and described in "Vegetal Polyurethane Resin Implant
Cranioplasty. Experimental Studies in Rabbits" by Luiz Fernando
Francisco, Sao Jose do Rio Preto, 1998) enough of the prepolymer and diol
(as indicated in Example 1) were mixed together to fill a 5 cc syringe.
The material was hand packed into the syringe and allowed to polymerize
for 18 hours at room temperature (air bubbles were noticed within the
sample). After polymerization was complete, the samples were removed from
the syringe and cut to length (approx. 13 mm). The results of mechanical
static compression tests, using the Bionix MTS 858 (Edin Prarrie Minn.),
are shown in column 5 of Table 2. The MPa values listed are only
approximate at the point of visible plastic deformation of the implant.
Samples did not mechanically fail at 20 MPa, but rather plastically
deformed such that the test had to be stopped at approximately 50%
strain. The load bearing capacity of the implants was still considered
acceptable for use as an implant.
TABLE-US-00002
TABLE 2
Approximate
Compressive
Sample ID Length (mm) Diameter (mm) Weight (g) Strength (MPa)
A-Dry 12.96 8.55 .78 20
B-Dry 13.97 8.52 .81 20

Example #3

[0069] To determine the compressive strength of a composite implant made
of approximately 75% bone and 25% castor bean polyurethane resin, 20
grams of bovine bone powder (particle size 120 .mu.m-500 .mu.m) were
combined with a 6.82 grams of a two part polyurethane (Doctors Research
Group, Plymouth Conn. and described in "Vegetal Polyurethane Resin
Implant Cranioplasty. Experimental Studies in Rabbits" by Luiz Fernando
Francisco, Sao Jose do Rio Preto, 1998). The mixture was then packed by
hand into three 5 cc syringes (packed with light hand pressure). The
samples were then set aside to polymerize over a 48-hour period at room
temperature. After polymerization was complete, the samples were removed
from the syringes and cut to length (approx. 14 mm). Of the 6 samples
tested; 4 were tested dry, while two were hydrated in Simulated Body
Fluid (SBF) for 24 hours and tested wet. The results of mechanical static
compression tests using the Bionix MTS 858 (Edin Prarrie Minn.) are shown
in column 5 of Table 3. Results indicated a decrease in compressive
strength (of about 21.8%) with the hydrated implants compared to the
compressive strength of the dry implants but load bearing capacity was
still considered acceptable for use as an implant.
TABLE-US-00003
TABLE 3
Compressive
Sample ID Length (mm) Diameter (mm) Weight (g) Strength (MPa)
A1-Dry 13.92 11.88 2.03 51
A2-Dry 14.02 11.87 2.14 56
A3-Wet 12.37 11.96 1.96 43
B1-Dry 14.16 11.86 2.25 59
B2-Dry 14.16 11.81 2.11 54
B3-Wet 14.34 11.92 2.23 43

Example #4

[0070] To determine if a composite implant could be made of bone with a
lysine diisocyanate and castor bean polyurethane resin; 6 grams of a
lysine diisocyanate were combined with 3.50 grams of a liquid comprising
castor bean oil fatty acid triglyceride "the diol". Next, the mixture was
heated to 93-95 degrees Celsius (on hot plate) and bone particles
(particle size 120 .mu.m-500 .mu.m) were gradually mixed into the
polyurethane solution, until the bone appeared well coated. The mixture
was then packed by hand into 5 cc syringes (packed with light hand
pressure). The samples were then set aside to polymerize over a 48-hour
period at room temperature. The material polymerized into a material that
could be extruded out of the syringe.

Example #5

[0071] 3 grams of lysine diisocyanate were mixed with ProGenix Carrier #2
and at least partially polymerized to produce a flexible gel like sheet
within a few hours.

Example #6

[0072] Tissue-derived materials are employed as chain extenders in
polyurethanes. Exemplary formulations are given in Table 4. Ratios of
polyol to isocyanate and of macropolyisocyanate to chain extender may be
varied according to specific requirements of the desired biomaterial over
a wide range, e.g., from about 10:1 to 1:10. A conventional
polymerization catalyst known to those skilled in the art (such as an
amine or tin compound) may or may not also be added, and the mixture is
then combined with the crosslinking agent indicated and placed in a mold
(such as Teflon) to polymerize. The percentage of the final composite
comprised of composite filler (i.e., bone) may be varied between 5% and
95% according to the specific requirements of the biomaterial. The
mixture polymerizes to form a bone-polyurethane composite. In one
embodiment, calcium phosphate granules are substituted for the bone
portion of the formulation. Exemplary preparations of calcium phosphates
are described by U.S. Pat. No. 5,650,176 to Lee et al., U.S. Pat. No.
6,002,065 to Constantz et al., and U.S. Pat. No. 6,206,957 to Driessens
et al., all of which are incorporated by reference herein. Additional
calcium phosphates that may be exploited for use with the invention
include but are not limited to .alpha.-tricalcium phosphate,
hydroxyapatite, dicalcium phosphate, .beta.-tricalcium phosphate,
tetracalcium phosphate, amorphous calcium phosphate, and octacalcium
phosphate. Substituted CaP phases are also contemplated for use with the
invention, including but not limited to fluorapatite, chlorapatite,
Mg-substituted tricalcium phosphate, and carbonate hydroxyapatite.
TABLE-US-00004
TABLE 4
Polyol/polyamine Chain extender
(proportions by weight %) Polyisocyanate (50% to 80% by weight)
Lecithin Hexamethylene Surface demineralized
Starch Diisocyanate bone particles
Starch:Lecithin 15:85 Uretdione (200-1000 microns)
Starch:Lecithin 85:15 polyisocyanate
Collagen
Polyactide Lysine Calcium phosphate
Poly (.epsilon.-caprolactone) diisocyanate
Hydroxy terminated ethyl ester
polyethelene oxide
Amine-terminated Lysine
poly(1,4-butadiene) diisocyanate
Tyrosine-based
polycarbonate
Polylysine Cyclohexyl- Cartilage
Polyserine diisocyanate
Polytyrosine Isocyanate
Glycerol terminated
Ethylene diamine polysaccharide

[0073] Other embodiments of the invention will be apparent to those
skilled in the art from a consideration of the specification or practice
of the invention disclosed herein. It is intended that the specification
and examples be considered as exemplary only, with the true scope and
spirit of the invention being indicated by the following claims.